An electronic device may be created from a workpieces that has undergone various processes. One of these processes may include introducing impurities or dopants to alter the electrical properties of the original workpiece. For example, charged ions, as impurities or dopants, may be introduced to a workpiece, such as a silicon wafer, to alter electrical properties of the workpiece. One process that introduces impurities to the workpiece may be an ion implantation process.
An ion implanter is used to perform ion implantation or other modifications of a workpiece. A block diagram of a conventional ion implanter is shown in FIG. 1. Of course, many different ion implanters may be used. The conventional ion implanter may comprise an ion source 102 that may be biased by a power supply 101. The system may be controlled by controller 120. The operator communicates with the controller 120 via user interface system 122. The ion source 102 is typically contained in a vacuum chamber known as a source housing (not shown). The ion implanter system 100 may also comprise a series of beam-line components through which ions 10 pass. The series of beam-line components may include, for example, extraction electrodes 104, a 90° magnet analyzer 106, a first deceleration (D1) stage 108, a 70° magnet collimator 110, and a second deceleration (D2) stage 112. Much like a series of optical lenses that manipulate a light beam, the beam-line components can manipulate and focus the ion beam 10 before steering it towards a workpiece or wafer 114, which is disposed on a workpiece support 116.
In operation, a workpiece handling robot (not shown) disposes the workpiece 114 on the workpiece support 116 that can be moved in one or more dimensions (e.g., translate, rotate, and tilt) by an apparatus, sometimes referred to as a “roplat” (not shown). Meanwhile, ions are generated in the ion source 102 and extracted by the extraction electrodes 104. The extracted ions 10 travel in a beam-like state along the beam-line components and implanted on the workpiece 114. After implanting ions is completed, the workpiece handling robot may remove the workpiece 114 from the workpiece support 116 and from the ion implanter 100.
Referring to FIGS. 2A and 2B, there is shown a block diagram illustrating the workpiece support 116 supporting the workpiece 114 during the ion implantation process. As illustrated in FIG. 2A, the workpiece support 116 may comprise a top layer 210 that is in contact with the workpiece 114. In addition, the workpiece support 116 may also include at least one cooling region 206. During the implantation process, cooling gas may be provided to the cooling region 206 prevent the workpiece 114 from overheating. The workpiece support 116 may have gas channels and conduits to allow this cooling gas to flow to the cooling region 206. The workpiece support 116 may further include a plurality of lift pins 208 that may move so as to push the workpiece 114 away from the workpiece support 116 in the direction indicated by the arrows. The lift pins 208 may be retracted within the workpiece support 116, as illustrated in FIG. 2B.
The workpiece support 116 may be cylindrical in shape, such that its top surface is circular, so as to hold a disc-shaped workpiece. Of course, other shapes, such as squares, are possible. To effectively hold the workpiece 114 in place, most workpiece supports typically use electrostatic force. By creating a strong electrostatic force on the upper side of the workpiece support 116, the support can serve as the electrostatic clamp or chuck, the workpiece 114 can be held in place without any mechanical fastening devices. This minimizes contamination, avoids wafer damage from mechanical clamping and also improves cycle time, since the workpiece does not need to be unfastened after it has been implanted. These clamps typically use one of two types of force to hold the substrate in place: coulombic or Johnsen-Rahbek force.
As seen in FIG. 2A, the workpiece support 116 traditionally consists of several layers. The first, or top, layer 210, which contacts the workpiece 114, is made of an electrically insulating or semiconducting material, such as alumina, since it must produce the electrostatic field without creating a short circuit. In some embodiments, this top layer 210 is about 4 mils thick. For those embodiments using coulombic force, the resistivity of the top layer 210, which is typically formed using crystalline and amorphous dielectric materials, is typically greater than 1014 Ω-cm. For those embodiments utilizing Johnsen-Rahbek force, the volume resistivity of the top layer 210, which is formed from a semiconducting material, is typically in the range of 1010 to 1012 Ω-cm. The term “non-conductive” is used to describe materials in either of these ranges, and suitable for creating either type of force. The coulombic force can be generated by an alternating voltage (AC) or by a constant voltage (DC) supply.
Directly below this layer is a conductive layer 212, which contains the electrodes that create the electrostatic field. This conductive layer 212 is made using electrically conductive materials, such as silver. Patterns are created in this layer, much like are done in a printed circuit board to create the desired electrode shapes and sizes. Below this conductive layer 212 is a second insulating layer 214, which is used to separate the conductive layer 212 from the lower portion 220.
The lower portion 220 is preferably made from metal or metal alloy with high thermal conductivity to maintain the overall temperature of the workpiece support 116 within an acceptable range. In many applications, aluminum is used for this lower portion 220.
Initially, the lift pins 208 are in a lowered position. The workpiece handling robot 250 then moves a workpiece 114 to a position above the workpiece support 116. The lift pins 208 may then be actuated to an elevated position (as shown in FIG. 2A) and may receive the workpiece 114 from the workpiece handling robot 250. Thereafter, the workpiece handling robot 250 moves away from the workpiece support 116 and the lift pins 208 may recede into the workpiece support 116 such that the top layer 210 may be in contact with the workpiece 114, as shown in FIG. 2B. The implantation process may then be performed with the lift pins 208 in this recessed position. After the implantation process, the workpiece 114 is unclamped from the workpiece support 116, having been held in place by electrostatic force. The lift pins 208 may then be extended into the elevated position, thereby elevating the workpiece 114 and separating the workpiece 114 from the top layer 210 of the workpiece support 116, as shown in FIG. 2A. The workpiece handling robot 250 may then be disposed under the workpiece 114, where it can retrieve the implanted workpiece 114 at the elevated position. The lift pins 208 may then be lowered, and the robot 250 may then be actuated so as to remove the workpiece 114 from the implanter.
This technique is effective, especially when the workpiece 114 and the workpiece support 116 are both substantially planar. This allows the workpiece 114 and workpiece support 116 to couple together closely when clamped. This tight coupling serves to confine the cooling gas to the cooling regions 206.
However, in some embodiments, the workpiece may not be planar. For example, it is advantageous for the surface of a solar cell to be textured, to minimize reflection of photons and thus maximize cell efficiency. One common method to achieve this textured surface is to bathe the workpiece in acid or alkaline solutions. While such baths are less expensive than other processes, they will texture both sides, not just the surface exposed to the photons. However, since manufacturing costs are critical for the solar cell industry, this may be an accepted consequence. Also ion implantation into the rear surface of the cell is beneficial in producing a back surface field, so even were only the front of the cell textured it would still be necessary to clamp the textured surface for this application.
One consequence of textured workpiece surfaces is that the workpiece support 116 and the workpiece 114 no longer form a tight coupling as described earlier. FIG. 3 shows an exaggerated view of the interface between a textured workpiece 200 and a workpiece support 116. This interface presents several issues related to the cooling of the workpiece 200. First, the textured surface of workpiece 200 implies that a lower percentage of the surface of the workpiece 200 is in physical contact with the workpiece support 116. This reduces the ability of the workpiece support 116 to pull heat away from the workpiece 200 via conduction. A second issue is related to the cooling gas. The workpiece 116 may have cooling conduits 210, as shown in FIG. 3. Gas is injected into the area between the workpiece 200 and the workpiece support 116, as described above, through the cooling conduits 210. However, since there is less contact between the textured workpiece 200 and the workpiece support 116, the gas is not confined to cooling regions (as described in connection to FIG. 2A). As result, the gas escapes from the edges between the textured workpiece 200 and the workpiece support 116. This increases the pressure within the chamber, which is preferably held as close to vacuum as possible, and decreases the pressure between workpiece and clamp. This is detrimental to the ion implantation process, and is detrimental in cooling the workpiece 200. A third issue is the lower available electrostatic clamp force due to the higher average gap.
Accordingly, there is a need in the art for an improved workpiece support that can effectively cool textured workpieces.